1. Field of the Invention
The present invention relates to an OLED (organic light emitting device) panel obtained by forming an OLED on a substrate and sealing the OLED between the substrate and a cover member. The invention also relates to an OLED module in which an IC including a controller, or the like, is mounted to the OLED panel. In this specification, ‘light emitting device’ is the generic term for the OLED panel and for the OLED module. Electronic equipment using the light emitting device is also included in the present invention.
2. Description of the Related Art
An OLED emits light by itself, and thus, has high visibility. The OLED does is not need a backlight necessary for a liquid crystal display device (LCD), which is suitable for a reduction of a light emitting device in thickness. Also, the OLED has no limitation on a viewing angle. Therefore, the light emitting device using the OLED has recently been attracting attention as a display device that substitutes for a CRT or the LCD.
The OLED includes a layer containing an organic compound in which luminescence generated by application of an electric field (electroluminescence) is obtained (organic light emitting material) (hereinafter, referred to as organic light emitting layer), an anode layer and a cathode layer. A light emission in returning to a base state from a singlet excitation state (fluorescence) and a light emission in returning to a base state from a triplet excitation state (phosphorescence) exist as the luminescence in the organic compound. The light emitting device of the present invention may use one or both of the above-described light emissions.
Note that, in this specification, all the layers provided between an anode and a cathode of the OLED are defined as the organic light emitting layers. The organic light emitting layers specifically include a light emitting layer, a hole injecting layer, an electron injecting layer, a hole transporting layer, an electron transporting layer and the like. The OLED basically has a structure in which an anode/a light emitting layer/a cathode are laminated in order. Besides this structure, the OLED may take a structure in which an anode/a hole injecting layer/a light emitting layer/a cathode are laminated in order or a structure in which an anode/a hole injecting layer/a light emitting layer/an electron transporting layer/a cathode are laminated in order.
The problem in putting a light emitting device into practice is lowering in luminance of OLED which accompanies degradation of an organic light emitting material.
Organic light emitting materials are weak against moisture, oxygen, light, and heat, which accelerate degradation of the organic light emitting materials. The rate of degradation of an organic light emitting material depends specifically on the structure of a device for driving the light emitting device, characteristics of the organic light emitting material, materials of electrodes, conditions in a manufacture process, how the light emitting device is driven, etc.
Even when the voltage applied to the organic light emitting layer is constant, the luminance of the OLED is lowered as the organic light emitting layer degrades, and an image to be displayed is therefore become unclearly. In this specification, a voltage applied to an organic light emitting layer from a pair of electrodes is called an OLED drive voltage (Ve1).
When an image is displayed in color by using three types of OLEDs that emit red (R) light, green (G) light, and blue (B) light, respectively, different organic materials are used to form organic light emitting layers of OLEDs of a plurality of colors. Accordingly, the rate of degradation of organic light emitting layer may vary between OLEDs of a plurality of colors. Then difference in luminance between OLEDs of a plurality of colors will be apparent as time passes, making it impossible for the light emitting device to display an image in desired colors.